Dr. Lixion Avila, senior hurricane specialist, foreground, updates Dr. Rick Knabb, director of the National Hurricane Center, on other developing tropical storms Wednesday, Aug. 22, 2012, at the National Hurricane Center in Miami. The U.S. National Hurricane Center on Wednesday morning said Isaac was expected to strengthen and could become a hurricane by Thursday. The storm is posing a potential threat to next week's Republican National Convention in Florida. (AP Photo/Alan Diaz)— AP

Dr. Lixion Avila, senior hurricane specialist, foreground, updates Dr. Rick Knabb, director of the National Hurricane Center, on other developing tropical storms Wednesday, Aug. 22, 2012, at the National Hurricane Center in Miami. The U.S. National Hurricane Center on Wednesday morning said Isaac was expected to strengthen and could become a hurricane by Thursday. The storm is posing a potential threat to next week's Republican National Convention in Florida. (AP Photo/Alan Diaz)
/ AP

This NOAA satellite image taken Thursday, Aug. 23, 2012 at 1:45 a.m. EDT shows Tropical Storm Isaac spinning over the eastern Caribbean Sea. The system is now about 265 miles southeast of San Juan, Puerto Rico with maximum winds up to 45 mph. A Hurricane Warning is in effect for Haiti and the Dominican Republic and a Tropical Storm Warning is in effect for most of the Caribbean Islands. The forecast storm track takes Isaac over Hispaniola and Cuba through the weekend. To the north, a frontal boundary lingers over the Southeast and Gulf of Mexico and produces more heavy rains and strong thunderstorms. In the Plains, a trough of low pressure pushes a front through the Upper Midwest and kicks up some widespread thunderstorm activity. A few of these storms may turn severe with strong winds and large hail. (AP PHOTO/WEATHER UNDERGROUND)— AP

This NOAA satellite image taken Thursday, Aug. 23, 2012 at 1:45 a.m. EDT shows Tropical Storm Isaac spinning over the eastern Caribbean Sea. The system is now about 265 miles southeast of San Juan, Puerto Rico with maximum winds up to 45 mph. A Hurricane Warning is in effect for Haiti and the Dominican Republic and a Tropical Storm Warning is in effect for most of the Caribbean Islands. The forecast storm track takes Isaac over Hispaniola and Cuba through the weekend. To the north, a frontal boundary lingers over the Southeast and Gulf of Mexico and produces more heavy rains and strong thunderstorms. In the Plains, a trough of low pressure pushes a front through the Upper Midwest and kicks up some widespread thunderstorm activity. A few of these storms may turn severe with strong winds and large hail. (AP PHOTO/WEATHER UNDERGROUND)
/ AP

SAN JUAN, Puerto Rico 
Puerto Rico and the U.S. Virgin Islands braced for torrential rains on Thursday as Tropical Storm Isaac churned waves as high as 10 feet (3 meters) in the Caribbean and threatened to become a hurricane that could take a shot at Florida just as Republicans gather for their national convention.

Some flooding was reported in eastern and southern regions of Puerto Rico as the storm approached.

U.S. forecasters said Isaac will likely turn into a Category 1 hurricane by Friday as it nears the Dominican Republic and Haiti. It was expected to weaken a little while heading over their island and Cuba, then possibly move on toward Florida as a hurricane by Monday.

Isaac was centered 225 miles (360) kilometers south-southeast of Puerto Rico Thursday morning, with maximum sustained winds of 40 mph (65 kph). It was moving west at 13 mph (21 kph) according to the U.S. National Hurricane Center.

The system has slowed down, and as a result, the island will see more rain, said Ernesto Morales, forecaster with the National Weather Service in San Juan, Puerto Rico.

"This is not the moment to become complacent," he said.

The U.S. territory has opened 428 shelters, and 50 people have taken refuge, said Gov. Luis Fortuno. Some 4,000 people were without power and more than 3,000 without water.

Schools and government offices remained closed Thursday, but the governor said it was safe for people to go to work if they needed to. However, he warned everyone to stay away from beaches and swollen rivers.

"It's not the day to participate in recreational activities in these areas," Fortuno said.

While Isaac itself has caused on reported injuries or deaths, police in Puerto Rico say a 75-year-old woman died near the capital of San Juan on Wednesday when she fell off a second-floor balcony while filling a drum with water in preparation for the storm.

Puerto Rico's main international airport remained open, but Cape Air and American Eagle cancelled all their flights Thursday, Fortuno said. Ferry service to the tourist islands of Vieques and Culebra also was temporarily suspended.

In Vieques, one of the owners of Bananas Guesthouse said he had received a call from his brother in Florida, who suggested he tell reporters "there are mudslides and cows flying through the air. But in fact, there's a breeze going by," Glenn Curry said. "We've had a little bit of rain. Nothing much has happened so far...Overnight it didn't even blow enough to wake me up."

In the U.S. Virgin Islands town of Christiansted, streets lined with historic buildings of Danish architecture, were largely deserted. All but a small handful of businesses and government offices were closed. Hurricane shutters covered the entrances to most buildings and sandbags were stacked in anticipation of potential floods and storm surge.

Two shelters were open on the island, and 10 people were housed overnight, according to Elton Lewis, director of the Virgin Islands Territorial Emergency Management Agency.

The storm already forced military authorities at the U.S. base in Guantanamo Bay, Cuba, to cancel pretrial hearings for five prisoners charged in the Sept. 11 attacks. They also planned to evacuate about 200 people, including legal teams and relatives of Sept. 11 victims.